Phosphorus ylides reactivities

The reactivity of the phosphorus ylide 1 strongly depends on substituents R R. For preparative use R often is a phenyl group. When R or R is an electron-withdrawing group, the negative charge can be delocalized over several centers, and the reactivity at the ylide carbon is reduced. The reactivity of the carbonyl compound towards addition of the ylide increases with the electrophilic character of the C=0 group. R R are often both alkyl, or alkyl and aryl. 

This review concerns in the first part the works published during the last three years on the synthesis and reactivity of stabilized ylides C-substituted by electron-withdrawing groups (COR, CO2R, CN, etc.). The second part deals with the works published in the same period on the chemistry of phosphorus ylides mainly C-substituted by heteroatoms of groups 1-16 (metals, metalloids and nonmetal elements Li, Ba, Ca, Ti, Zr, Nb, Mo, Re, Fe, Ru, Rh, Pd, Pt, Au, Zn, Hg, B, Si, Sn, N, P, As, Sb, O, S, Te). 

For group 14, C-tin-substituted phosphorus ylides have been studied in the past but less than the corresponding C-silyl ylides which, owing to their stability and reactivity, are of considerable interest. Indeed a recent review concerning the silylphosphanes with one part concerning the synthesis and applications of silylated phosphorus ylides has been published [112]. 

The second chapter (M. Taillefer and H. J. Cristau) is dedicated to new trends in ylide chemistry. The preparation and the reactivity of phosphorus ylides, C-substituted by heteroatoms is presented, ylides being substituted by groups 1 and 2 elements, by transition metals or by elements of groups 13 to 16. A rich and versatile chemistry is thus reported. 

Interestingly, cyclopropenone exhibited comparable reactivity towards sulfur ylide 437 and phosphorus ylide 439 giving rise to 6-phenyl-2-pyrone and a-naphthol, respectively197. Again, the intermediacy of ketenes 438f440 may reasonably explain the formation of these products. 

Because of the presence of a lone pair and a vacant orbital, singlet carbenes are supposed to be able to react with both Lewis bases and acids. Transient electrophilic carbenes are known to react with Lewis bases to give normal ylides (Scheme 8.19). For example, carbene-pyridine adducts have been spectroscopically characterized and used as a proof for the formation of carbenes,and the reaction of transient dihalogenocarbenes with phosphines is even a preparative method for C-dihalogeno phosphorus ylides. Little is known about the reactivity of transient carbenes with Lewis acids. 

The reactivity of phosphorus ylides towards carbonyl compounds such as ketones, esters, carbonates and amides has also been explored13. The mechanism shown in... 

Following Schlosser (ref. 9), we divided the alkylidene phosphoranes (phosphorus ylides) 1. used in Wittig reactions into three groups according to their reactivity. 

Benzene- and naphthalene-annulated tetracoordinated phosphetes have been synthesized and their reactivities toward insertion reactions were investigated. As expected, benzophosphete 49 is very reactive because it contains a phosphorus ylide moiety. Insertion of dimethyl acetylenedicarboxylate and acetonitrile occurred at room... 

These derivatives have not only provided new synthetic pathways but have shown improved thermal stability (as in the case of arsenic ylides) and a modified pattern of chemical reactivity. The donor properties of ylides 55, 24), and most of their synthetic applications 103), have been covered in other reviews and articles 3, 26) and are not duplicated here. The general organometallic chemistry of arsenic, antimony, and bismuth is the subject of the invaluable monograph by Doak and Freedman 11). The broad scope of phosphorus ylide and pentaorganophosphorane chemistry was covered in the leading multivolume series on organophosphorus chemistry edited by Kosolapoff and Maier 3, 21). Finally, the recent... 

There are essentially three different types of transition metal carbene complexes featuring three different types of carbene ligands. They have all been named after their first discoverers Fischer carbenes [27-29], Schrock carbenes [30,31] and WanzUck-Arduengo carbenes (see Figure 1.1). The latter, also known as N-heterocycUc carbenes (NHC), should actually be named after three people Ofele [2] and Wanzlick [3], who independently synthesised their first transition metal complexes in 1968, and Arduengo [1] who reported the first free and stable NHC in 1991. Fischer carbene complexes have an electrophilic carbene carbon atom [32] that can be attacked by a Lewis base. The Schrock carbene complex has a reversed reactivity. The Schrock carbene complex is usually employed in olefin metathesis (Grubbs catalyst) or as an alternative to phosphorus ylides in the Wittig reaction [33]. 

Stabilized Ylides.The carbanion center of phosphorus ylides is further stabilized by conjugation, usually with a carbonyl group (-CHO, -COR, -CO2R). Hence, these ylides are less reactive and weaker bases, but are more stable toward oxygen and pro-tic solvents than nonstabilized ylides. This type of stabilized ylide is mainly used in olefmations with aldehydes. 

The ambidentate nature of phosphinomethanides I is well documentated. They may react with electrophiles either via the carbon or the phosphorus atom, forming heteroelement substituted methanes or phosphorus ylides. The reactivity of the phosphinomethanides can be tuned by the choice of P and C substituents R, X, and Y and by the specific reaction conditions. 

The past year has seen continued interest in the structural nature of phosphorus ylides with the aim of gaining greater insight into their stabilities, electronic distributions and conformations, which ultimately affect the reactivities of these species. However, there have been fewer mechanistic studies of the Wittig reaction itself although several studies into the closely related aza-Wittig and Horner-Wadsworth-Emmons modifications have appeared. 

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